Atomic Structure Class 11th JEE

What is Atom?

An atom is a particle of matter that exceptionally characterizes a chemical element. An atom comprises a focal nucleus that is encircled by at least one adversely charged electron. The nucleus is emphatically charged and contains at least one moderately weighty particle known as protons and neutrons.

Atoms are the essential structural blocks of matter. Anything that occupies room and anything with mass is comprised of atoms.

Dalton’s Atomic Theory 

Dalton's atomic theory was a logical theory on the idea of matter set forward by the English physicist and chemist John Dalton in the year 1808. It expressed that everything matter was comprised of little, unbreakable particles known as 'atoms'.

Postulates of Dalton’s Atomic Theory

• The matter is comprised of small, unbreakable particles called atoms.
• All atoms of a particular element are indistinguishable in mass, size, and different properties. Be that as it may, atoms of various elements display various properties and fluctuate in mass and size.
• Atoms can neither be made nor annihilated. Moreover, atoms can't be isolated into more modest particles.
• Atoms of various elements can consolidate in fixed entire number proportions to shape compounds.
• Atoms can be improved, consolidated, or isolated in chemical responses.

Limitations of Dalton’s Atomic Theory

• It doesn't represent subatomic particles: Dalton's atomic theory expressed that atoms were unified. Nonetheless, the disclosure of subatomic particles (like protons, electrons, and neutrons) refuted this proposal.
• It doesn't represent isotopes: According to Dalton's atomic theory, all atoms of an element have indistinguishable masses and densities. Be that as it may, various isotopes of elements have different atomic masses (For instance hydrogen, deuterium, and tritium).
• It doesn't represent isobars: This theory expresses that the masses of the atoms of two unique elements should vary. Nonetheless, two unique elements can have a similar mass number. Such atoms are called isobars (Model: 40Ar and 40Ca).
• Elements need not join in straightforward, entire number proportions to shape compounds: Certain perplexing natural mixtures don't highlight the basic proportions of constituent atoms. Model: sugar/sucrose (C₁₁H₂₂O₁₁).
• The theory doesn't represent allotropes: The distinctions in that frame of mind of jewel and graphite, the two of which contain no one but carbon, can't be made sense of by Dalton's atomic theory.

Charge to Mass Ratio

J. J. Thomson, in the nineteenth century, proposed the Thomson Atomic Model which found the electron to denote the origin of subatomic particles. After the disclosure of the electron, he went on with his examinations for computing the mass and the charge of the electron. With the assistance of these estimations, he made a determined equation to compute the charge to the mass proportion of electrons. In this article, we will concentrate on the mass-to-charge proportion and the computation of the charge-by-mass ratio.

Charge to Mass Ratio of an Electron

e/m = 1.758820 × 10¹¹ C/kg

Where in,

m = mass of electron in kg = 9.10938356 × 10^-31 kilograms.

e  = magnitude of the charge of the electron in coulombs= 1.602 × 10^-19 coulombs.

Mass on Electron

The mass of a stationary electron is known as the electron mass. It is otherwise called the electron's invariant mass and is one of material science's key constants. It weighs around 9.1091031 kilograms or 5.486104 daltons. The mass of an electron relates to an energy of around 8.1871014 joules or around 0.5110 MeV.

Neutron and its Discovery

The British physicist Sir James Chadwick discovered neutrons in the year 1932. He was awarded the Nobel Prize in Physics in the year 1935 for this discovery.

How it was discovered:

• James Chadwick fired alpha radiation at a beryllium sheet from a polonium source. This led to the production of uncharged, penetrating radiation.
• This radiation was made incident on paraffin wax, a hydrocarbon having a relatively high hydrogen content.
• The protons ejected from the paraffin wax (when struck by the uncharged radiation) were observed with the help of an ionization chamber.
• The range of the liberated protons was measured and the interaction between the uncharged radiation and the atoms of several gases was studied by Chadwick.
• He concluded that the unusually penetrating radiation consisted of uncharged particles having (approximately) the same mass as a proton. These particles were later termed ‘neutrons’.

Thompson Model of Atom

Thomson's atomic model was proposed by William Thomson in the year 1900. This model explained the description of the inner structure of the atom theoretically. It was strongly supported by Sir Joseph Thomson, who had discovered the electron earlier.

During the cathode ray tube experiment, a negatively charged particle was discovered by J.J. Thomson. This experiment took place in the year 1897. A cathode ray tube is a vacuum tube. The negative particle was called an electron.

Thomson assumed that an electron is two thousand times lighter than a proton and believed that an atom is made up of thousands of electrons. In this atomic structure model, he considered atoms surrounded by a cloud having positive as well as negative charges. The demonstration of the ionization of air by X-ray was also done by him together with Rutherford. They were the first to demonstrate it. Thomson’s model of an atom is similar to a plum pudding.

Postulates of Thomson’s atomic model

Postulate 1: An atom consists of a positively charged sphere with electrons embedded in it

Postulate 2: An atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude

Thomson's atomic model is compared to watermelon. Where he considered the:

• Watermelon seeds as negatively charged particles
• The red part of the watermelon is positively charged

Limitations of Thomson’s atomic model

• It failed to explain the stability of an atom because his model of the atom failed to explain how a positive charge holds the negatively charged electrons in an atom. Therefore, This theory also failed to account for the position of the nucleus in an atom
• Thomson’s model failed to explain the scattering of alpha particles by thin metal foils
• No experimental evidence in its support

Rutherford’s model of Atom

Rutherford proposed the atomic structure of elements. According to the Rutherford atomic model:

1. The positive charge and most of the mass of an atom are concentrated in an extremely small volume. He called this region of the atom a nucleus.
2. Rutherford’s model proposed that the negatively charged electrons surround the nucleus of an atom. He also claimed that the electrons surrounding the nucleus revolve around it at very high speed in circular paths. He named these circular paths orbits.
3. Electrons being negatively charged and the nucleus being a densely concentrated mass of positively charged particles are held together by a strong electrostatic force of attraction.

Limitations of the Rutherford Atomic Model

Although the Rutherford atomic model was based on experimental observations, it failed to explain certain things.

• Rutherford proposed that the electrons revolve around the nucleus in fixed paths called orbits. According to Maxwell, accelerated charged particles emit electromagnetic radiation and hence an electron revolving around the nucleus should emit electromagnetic radiation. This radiation would carry energy from the motion of the electron which would come at the cost of shrinking of orbits. Ultimately the electrons would collapse in the nucleus. Calculations have shown that as per the Rutherford model, an electron would collapse into the nucleus in less than 10-8 seconds. So the Rutherford model was not by Maxwell’s theory and could not explain the stability of an atom.
• One of the drawbacks of the Rutherford model was also that he did not say anything about the arrangement of electrons in an atom which made his theory incomplete.

Although the early atomic models were inaccurate and failed to explain certain experimental results, they formed the base for future developments in the world of quantum mechanics.

Isotopes

Atoms with the same number of protons but different numbers of neutrons are called isotopes. They share almost the same chemical properties, but differ in mass and therefore in physical properties. There are stable isotopes, which do not emit radiation, and there are unstable isotopes, which do emit radiation. The latter are called radioisotopes.

Isobars

Isobar is an element that differs in chemical properties, but it has similar physical properties. Hence, we can say that isobars are elements that have different atomic numbers but the same mass number. Also, they have a different chemical properties because there is a difference in the electron count. An isobar contains the same atomic mass but a different atomic number because an added number of neutrons recompense the number of nucleons.

Bohr’s Model and its applications

Bohr model is one of the first things you learn because it improvised and changed many concepts in the right direction. This model was initially accepted as the rectification of the very famous Rutherford Model. Coined by Niels Bohr in the year 1913, the model took into account the structure and mechanism of the Solar system. The planets were replaced by the orbiting electrons, the Sun is analogous to a dense nucleus, and the gravitational pull by electrostatic force.

Limitations of the Bohr Model

• The model explained the Hydrogen atom smoothly but it failed to explain complex atoms having atomic numbers much higher than the hydrogen atom.
• It only explained the occurrence of the line spectrum but it couldn’t explain their intensity. Some lines were found to be more intense than others without any explanation.
• Upon closer inspection of the spectra, it was found that there was even more splitting into the lines obtained for any atom. There was no explanation for the fine structure splitting or hyperfine structure of the lines in the Bohr model.
• There was a direct contradiction of Heisenberg’s uncertainty principle because Bohr’s model certainly claimed to predict the radius and velocity of an electron in a certain orbit using the value of n.

Plank’s Quantum Theory

According to Planck’s quantum theory,

1. Different atoms and molecules can emit or absorb energy in discrete quantities only. The smallest amount of energy that can be emitted or absorbed in the form of electromagnetic radiation is known as quantum.
2. The energy of the radiation absorbed or emitted is directly proportional to the frequency of the radiation.

Meanwhile, the energy of radiation is expressed in terms of frequency as,

E = h ν

Where,

E = Energy of the radiation

h = Planck’s constant (6.626×10^-34 J.s)

ν= Frequency of radiation

Curiously, Planck has additionally presumed that these were just a part of the cycles of ingestion and emanation of radiation. They didn't have anything to do with the actual truth of the actual radiation. Later in the year 1905, renowned German physicist, Albert Einstein additionally reconsidered Planck's theory to make sense of the photoelectric impact. He was of the assessment that assuming some wellspring of light was centered around specific materials, they can discharge electrons from the material. Essentially, Planck's work drove Einstein in confirming that light exists in discrete quanta of energy, or photons.

Photoelectric Effect

The photoelectric effect is a peculiarity wherein electrons are launched out from the outer layer of a metal when light is occurring on it. These shot-out electrons are called photoelectrons. It is critical to take note that the outflow of photoelectrons and the active energy of the catapulted photoelectrons are subject to the recurrence of the light that is an episode on the metal's surface. The interaction through which photoelectrons are launched out from the outer layer of the metal because of the activity of light is regularly alluded to as photoemission.

The photoelectric effect was first presented by Wilhelm Ludwig Franz Hallwachs in the year 1887 and the trial check was finished by Heinrich Rudolf Hertz. They saw that when a surface is presented to electromagnetic radiation at a higher limit recurrence, the radiation is consumed and the electrons are transmitted. Today, we concentrate on the photoelectric effect as a peculiarity that includes a material retaining electromagnetic radiation and delivering electrically charged particles.

Dual Nature of Matter

The dual nature of matter is a significant idea in JEE Chemistry and is fundamentally the investigation of various natures that matter has or displays. A matter can either show or have a particle nature or wave nature. Different experiments have additionally been led to demonstrate this theory.

At first, the properties of matter or light were made sense of regarding its particle nature. Corpuscular theory of light, and so forth were a portion of the crude advances that impacted this. Later on, it was tentatively figured out that matter has the properties of a wave. Thus, the matter is said to have dual nature, i.e., it has both the properties of a particle and as well as a wave.

Maxwell's equation of electromagnetism and Hertz's experiments on the age and location of electromagnetic waves in 1887 firmly settled the wave nature of light.

Hence, the wave-particle duality is a significant idea in quantum mechanics which portrays that each particle or all the more explicitly quantum substance might be communicated as either a particle or a wave. This idea further assists with altering the powerlessness of the traditional repairman's methodology or hypotheses to depict the way of behaving of the matter completely.

Upcoming Topics

• De-Broglie’s Waves
• Heisenberg’s Uncertainty Principle
• Spectrum
• Rydberg’s Constant
• Quantum mechanical model of Atom
• Concept of orbitals
• Quantum numbers
• Rules for filling subshell
• Exclusion principle

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SOME BASIC CONCEPTS OF CHEMISTRY CLASS 11TH JEE